Rpc: Introduce call for per-block fee statistics

[salisbury-espinosa]

original: ElementsProject/elements#185
related:
RPC: Introduce getblockstats to plot things bitcoin#10757
Qt: Show transaction size in transaction details window bitcoin#8672

[ega-forever]

Are there any updates on "getblockstats" API?, I need it as well

[PastaPastaPasta]

bitcoin#10757 backported by #3058 and bitcoin#8672 was backported by #1615